1. Field of the Invention
The present invention relates to a waveform equalizer circuit of an optical information recording and regenerating apparatus which can optically record and regenerate information to a recording medium.
2. Description of the Related Art
A recording and regenerating system of an optical disc has a transmission characteristics which is called “optical transfer function (OTF)” determined by a laser wavelength and a numerical aperture of a lens in an optical pickup. This OTF has a characteristic of a kind of low pass filter. Accordingly, when the recording density of data recorded on the optical disc is increased to increase a recording capacity of the optical disc, the transmission band becomes short and hence, when neighboring marks are to be regenerated, interference between signs in which respective regenerating waveforms interfere with each other is generated. As a technique for attenuating this interference between signs, a waveform equalizer circuit which emphasizes high-band components of regenerating signals is used. However, since the optical transmission characteristics are changed depending on the relationship between the disc and a pickup, when the equalization characteristics are fixed, the interference between signs is generated due to factors such as a tilt of the disc and the regenerating signals are deteriorated. Further, since the band of the optical transmission characteristics is changed depending on the regenerating speed of the disc, in the variable-speed regeneration such as a CAV or the like, it becomes necessary to change the equalization characteristics following the regenerating speed of the disc. To solve such a problem, the waveform equalizing technique which employs an adaptive equalizer circuit is used. The adaptive equalizer circuit changes equalization characteristics corresponding to the change of the transmission characteristics in an input signal system and transmits proper signals to an output signal system.
A conventional example of the adaptive equalizer circuit is shown in FIG. 2. A sample value input 201 which is obtained by sampling reading signals 200 read by an optical disc not shown in the drawing by means of a sample hold circuit 205 is inputted to a system which is comprised of n pieces of unit delay elements Dl-Dn which are connected with each other in the longitudinal direction. The unit delay elements Dl-Dn have a time delay equal to a sampling period of the above-mentioned sample values and an output of one unit delay element becomes an input of one preceding sampling. In multiplication circuits MO-Mn, products of the signal 201 and the sample values outputted from respective delay elements and coefficients computed by coefficient control circuits CO-Cn are computed and the products are inputted into an addition circuit 203. An output from the addition circuit 203 is outputted as an output value 202 of the adaptive equalizer circuit and at the same time is inputted to a subtraction circuit 204. In the subtraction circuit 204, the difference between an output value Vo and an arbitrary given reference value is outputted as an adaptive error value. This reference value is determined such that the equalization characteristics of this adaptive equalizer circuit become the targeted transmission characteristics. This determination method is explained later in detail. The error value obtained by the subtraction circuit 204 is inputted to the coefficient control circuits CO-Cn. Each coefficient control circuit is constituted by a multiplication circuit and an integration circuit. For example, in the coefficient control circuit CO, the product of the input sample value 201 and the above-mentioned error value is computed by the multiplication circuit LO and the obtained value is averaged out by the integration circuit SO and is outputted to the multiplication circuit MO as a coefficient.
In this manner, by sequentially updating the coefficients of a FIR (Finite Impulse Response) filter, the adaptive equalizer circuit sets the equalization characteristics to the targeted transmission characteristics.
Subsequently, the above-mentioned reference values are explained. Here, as an input to the adaptive equalizer circuit, for example, a signal shown in FIG. 3 is considered. In this waveform, the sample value in the vicinity of a zero-crossing point indicated by numeral 301 becomes 0 when transmission characteristics are properly equalized. Accordingly, a sample which has the output Vo of the adaptive equalizer circuit in the vicinity of the zero-crossing point is extracted, and then, the difference between the above-mentioned Vo and the reference value is computed while assuming the reference value as 0, and the computed value is inputted to the coefficient control circuit as the equalizer error whereby a proper equalizer coefficient to the input waveform shown in FIG. 3 can be obtained. Further, as another technique for setting the reference value, as shown in FIG. 4, threshold values +Vth and −Vth are set and the comparison of magnitude between the output Vo of the adaptive equalizer circuit and the threshold value is performed and the reference value is changed based on the result.
For example, with respect to an example shown in FIG. 4, when the output Vo of the adaptive equalizer circuit is set to Vo<−Vth, the reference value is set to −1, when the output Vo of the adaptive equalizer circuit is set to −Vth<Vo<Vth, the reference value is set to 0, and when the output Vo of the adaptive equalizer circuit is set to Vth<Vo, the reference value is set to 1.
Due to such a constitution, it becomes possible to perform the updating of coefficients with respect to all output values of the adaptive equalizer circuit so that the extraction of the output values in the vicinity of the zero-crossing point becomes unnecessary.
The setting of these reference values and the manner of operation of the adaptive equalizer circuit are described in detail in Japanese Laid-open Publication 321671/1997.